The present invention relates to multilayer coextruded light-reflecting films which have a narrow reflection band due to light interference. When the reflection band occurs within the range of visible wavelength, the film is iridescent. Similarly, when the reflection band falls outside the range of visible wavelength, the film is either ultraviolet or infrared reflecting. Such multilayer films and methods by which they can be produced are known in the art. They are described, for instance, in U.S. Pat. Nos. 3,565,985, 3,759,657, 3,773,882 and 3,801,429 and other patents.
The multilayer films are composed of a plurality of generally parallel layers of transparent thermoplastic resinous material in which the contiguous adjacent layers are of diverse resinous material whose index of refraction differs by at least about 0.03. The film contains at least 10 layers and more usually at least 35 layers and, preferably, at least about 70 layers.
The individual layers of the film are very thin, usually in the range of about 30 to 500 nm, preferably about 50-400 nm, which causes constructive interference in light waves reflected from the many interfaces. Depending on the layer thickness and the refractive index of the polymers, one dominant wavelength band is reflected and the remaining light is transmitted through the film. The reflected wavelength is proportional to the sum of the optical thickness of a pair of layers.
The quantity of the reflected light (reflectance) and the color intensity depend on the difference between the two refractive indices, on the ratio of optical thicknesses of the layers, on the number of layers and on the uniformity of the thickness. If the refractive indices are the same, there is no reflection at all from the interfaces between the layers. In multilayer iridescent films, the refractive indices of contiguous adjacent layers differ by at least 0.03 and preferably by at least 0.06 or more. For first order reflections, reflectance is highest when the optical thicknesses of the layers are equal, although suitably high reflectances can be achieved when the ratio of the two optical thicknesses falls between 5:95 and 95:5. Distinct color reflections are obtained with as few as 10 layers. However, for maximum color intensity it is desired to have been 35 and 1,000 or even more layers. High color intensity is associated with a reflection band which is relatively narrow and which has high reflectance at its peak. It should be recognized that although the term "color intensity" has been used here for convenience, the same considerations apply to the invisible reflection in the ultraviolet and infrared ranges.
The multilayer films can be made by a chill-roll casting technique using a conventional single manifold flat film die in combination with a feedblock which collects the melts from each of two or more extruders and arranges them into the desired layer pattern. Feedblocks are described for instance in U.S. Pat. Nos. 3,565,985 and 3,773,882. The feedblocks can be used to form alternating layers of either two components (i.e. ABAB . . . ); three components (e.g. ABCABCA . . . or ACBCACBC . . . ); or more. The very narrow multilayer stream flows through a single manifold flat film die where the layers are simultaneously spread to the width of the die and thinned to the final die exit thickness. The number of layers and their thickness distribution can be changed in inserting a different feedblock module. Usually, the outermost layer or layers on each side of the sheet are thicker than the other layers. This thicker skin may consist of one of the components which makes up the optical core; may be a different polymer which is utilized to impart desirable mechanical, heat sealing, or other properties; or may be a combination of these.
Some recent developments in the iridescent film are described in U.S. Pat. Nos. Re. 31,780; 4,937,134; and 5,089,318. U.S. Pat. No. Re. 31,780 describes using a thermoplastic terephthalate polyester or copolyester resin as the high refractive index component of the system. Formation of elastomeric interference films are described in U.S. Pat. No. 4,937,134 in which all of the resinous materials are certain thermoplastic polyurethanes, polyester block amides or flexible copolyesters. U.S. Pat. No. 5,089,318 discloses improved multilayer light-reflecting transparent thermoplastic resinous film of at least 10 generally parallel layers in which the contiguous adjacent layers are of diverse transparent thermoplastic resinous material differing in refractive index by at least about 0.03 and at least one of the resinous materials being an engineering thermoplastic elastomer resin.
It has been desired to incorporate color into these iridescent films in order to add a new dimension to their appearance. Such colors can enhance or change the reflection or transmission colors of the iridescent film. In addition, while iridescent colors change with the viewing angle, the non-iridescent colors remain the same and hence the colors that can be observed can and will change dramatically based on the combination of the iridescent and non-iridescent component. Unfortunately, incorporation of the non-iridescent color into the iridescent film has proven to be elusive. In discussing colorants for plastics it is important to make a distinction between dyes and pigments. Under the prevailing processing conditions, pigments are virtually insoluble in plastics, whereas dyes are soluble.
Attempts to incorporate various pigments, both of a pearlescent and non-pearlescent nature, did not give rise to satisfactory results. It is now believed that the reason for these poor results was due to one or more of the following reasons: For the first order colors, which are the brightest, the layers in the optical core of the film usually have a thickness of about 0.03 to 0.2 micron while the pigment particle size is usually in the range of about 0.3 micron. This means that the pigments are larger than the layers and their use disrupts the interfaces between the layers which in turn results in the loss of iridescence and light scattering. The use of pigments whose particle size is less than that of the layer thickness in the optical core has resulted in agglomeration and aggregation during processing of the film resulting in the formation of a color body whose particle size was greater than the optical core layer thickness. In those instances in which such aggregation did not occur, one of two equally undesirable results was encountered. Either the pigment concentration was inadequate to realize any significant effect or when the pigment was incorporated in a concentration sufficient to contribute significantly to the appearance of the final film, the characteristics of the resin had been changed to such an extent that a co-extruded film could not be made. An attempt to overcome this problem was made by incorporating the pigment into the skin layer of the film which typically was in the range of 3 to 7 microns in thickness and comprised 20 to 25% of the total film thickness. Here also, up to a given pigment concentration, the contribution of the pigment was inadequate and was overpowered by the iridescent colors so the film appeared as if no pigment had been added and when this concentration was exceeded, the loading levels were found to be too high for the resins to be drawn down to be cast into film.
It has now been discovered that the foregoing problem can be overcome if a transparent dye having certain characteristics is incorporated into the resinous material of the layers.